As the transmission rates of serial digital data steadily increase, the synchronization between the transmitter and remote receivers becomes more complex, expensive and error prone. High bit rates of 50 megabits and greater require sophisticated circuits with components of precise values and redundant checking techniques. As the bit rates approach the switching limits of the decoder, the internal noise of the decoder becomes proportionately higher. Heretofore, at the lower bit rates there was an opportunity to sample the transmitted data frequently each bit time but the current rates preclude that technique. Therefore, the noise encountered during both transmission and decoding is a more significant factor and necessitates extensive countermeasures.
In the transmission of signals such as the Manchester code, the typical receiver uses fixed frequency oscillators to sample the input wave form. Most of these are difficult to operate, and they generate an excessive number of errors. The use of phase-locked loops is a related technique of demodulation but this is unsuitable for data bus operation which requires multiplexing since multiple receivers and transmitters are attached to the data bus.